Method of scheduling requests to banks in a flash controller

ABSTRACT

A memory system includes a memory controller, a first memory bank having a first I/O bus, a second memory bank having a second I/O bus, and a channel connecting the first I/O bus and the second I/O bus to the memory controller. The channel is used to transmit data between the first memory bank and the memory controller and between the second memory bank and memory controller, and is also used to transmit a command from the memory controller to the first memory bank and the second memory bank. The memory controller includes a bank command scheduler implemented in a hardware logic block. The hardware logic block includes a plurality of direct inputs and is able to determine, based on the plurality of inputs, an order in which to output commands to the first memory bank and the second memory bank over the channel; output a first command to the first memory bank; and output a second command to the second memory bank over the channel.

FIELD OF THE INVENTION

The present invention generally relates to systems and methods toschedule transmission of commands over a shared channel to banks of NANDdevices in a memory system.

BACKGROUND OF THE INVENTION

A solid state device (SSD) includes a plurality of NAND devicesorganized into banks which process commands and operations. SSDs includea scheduler which determines an order in which commands, including read,write, and erase commands, should be sent to the banks such that overallprogress is made on the commands in the queues. This includes ensuringthat low-latency operations such as reads are not queued behindlonger-latency operations such as erases, and ensuring that the order ofrequests from application block requests, meta-data requests, andgarbage collection requests are appropriately handled. In some cases,the scheduler manages a power consumption of the device as a whole.Since each of reads, writes, and erases consume different power, theoperation scheduler may control overall device power consumption bycontrolling how many of each operation is active at a time.

Traditionally, command scheduling systems are implemented with anindividual scheduler assigned to each bank of devices. The schedulerdetermines the optimal arrangement of priority and normal prioritycommands to be transmitted to the bank of devices. The schedule ofcommands transmitted to the bank of devices allows the system to moreefficiently transmit and execute commands.

Multiple banks of NAND devices can be driven in parallel to increase thecommand transfer efficiency. One parallel operating method is atechnique for interleaving a series of commands to a plurality of banksof NAND devices connected to the same channel (known as “bankinterleaving”). The process of bank interleaving increases theefficiency of the execution of commands on the SSD by optimizing theusage of the channel and allows the banks to process commands inparallel.

Bank interleaving maximizes the commands sent to the banks in a givenamount of time. For a command is that includes a latency time (forexample, a write command), the time that the system waits for theresponse from the bank would be unused without bank interleaving. Thiscontributes to an inefficiency of the system.

In systems which make use of bank interleaving, a scheduler sends afirst command including a sequence of instructions to a bank over achannel. The scheduler then determines an efficient use of the latencytime associated with the sequence of instructions and sends additionalcommands to other banks over a common channel. An operation schedulerschedules read, write, and erase operations within banks of devices overa memory channel or memory channel bus. The operation scheduler mayreceive various inputs including the operations currently being executedand operations in the command or operations queue.

Bank interleaving is typically implemented using a software based logicsystem in which a command scheduler repeatedly iterates through a seriesof loops in order to determine an optimal schedule of commands. However,using software logic to determine a schedule of interleaved commandsquickly becomes bulky and time delays are introduced when scaled up dueto the number of inputs required to efficiently schedule the commands.For example, the determination of an optimal schedule of commands relieson knowledge of a large number of variables including informationrelated to the status of each bank of devices, the command currentlybeing executed by each bank, the time that each of the command will taketo be completed, and a status of each of the queues of commands waitingto be transmitted to the banks. While a software-based loop of code caniterate through this information for a small number of banks todetermine a next command to be transmitted, as the number of bankssharing a channel is increased the code becomes unwieldy and theprocessing time for each determination suffers. Accordingly, there is along-felt need to correct the problems inherent to present day systems.

BRIEF DESCRIPTION OF THE INVENTION

In an aspect, a memory system includes a memory controller, a firstmemory bank having a first I/O bus, a second memory bank having a secondI/O bus, and a channel connecting the first I/O bus and the second I/Obus to the memory controller. The channel is used to transmit databetween the first memory bank and the memory controller and between thesecond memory bank and the memory controller. The channel is also usedto transmit a command from the memory controller to the first memorybank and the second memory bank. The memory controller includes a bankcommand scheduler implemented in a hardware logic block, and thehardware logic block includes a plurality of direct inputs. The hardwarelogic block determines, based on the plurality of direct inputs, anorder in which to output commands to the first memory bank and thesecond memory bank over the channel, outputs a first command to thefirst memory bank, and outputs a second command to the second memorybank over the channel.

In another aspect, a method of scheduling a distribution of commands toa first and a second memory bank includes using a hardware logic blockto determine a first command to send to a first NAND bank over a channeland transmitting the first command to the first NAND bank over thechannel. The method also includes using the hardware logic block todetermine from a plurality of direct hardware inputs a waiting time forthe first command to be executed by the first NAND bank, determining asecond command to send to a second NAND bank over the channel during thewaiting time for the first command to be executed, receiving acompletion signal from the first NAND bank related to the first command,and prior to receiving the completion signal, transmitting the secondcommand to the second NAND bank over the channel.

In another aspect, a method of determining a bank scheduling order for aNAND device having a plurality of banks with a shared channel includessetting an input register to a first priority rule, and setting anoperation request to determine a first status of the plurality of banksof the NAND device and a plurality of command queues. The method furtherincludes determining whether the first priority rule and the operationrequest is sufficient to select a bank to receive a command. If a bankcan be selected to receive the command according to the first priorityrule and the operation request, checking an operation status for thebank scheduled to receive the command and transmitting the command tothe bank when the operation status is not busy. If a bank cannot beselected to receive the command according to the first priority rule andthe operation request, the method includes setting the input register toa second priority rule for, setting an operation request to determine asecond status of the plurality of banks of the NAND device and theplurality of command queues, determining that a next bank to bescheduled can be determined from the operation request and the secondpriority rule, and transmitting the command to the selected bank.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a block diagram of an SSD including a bank scheduler withdecision logic, according to an embodiment;

FIG. 2 shows a block diagram of a single channel's bank scheduler withdecision logic, according to an embodiment;

FIG. 3 shows a block diagram of the inputs to the bank scheduler,according to an embodiment;

FIG. 4 shows a flow chart of a process for scheduling queue items usinga bank scheduler with hardware support logic, according to anembodiment; and

FIG. 5 shows a timing chart illustrating the scheduling of commands tomultiple banks over a shared channel.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a block diagram 100 that schematically illustrates thestructure of a solid-state drive (SSD) 102, in accordance with anembodiment of the present invention. The SSD 102 couples to a hostdevice 112.

The host device 112 (e.g., a personal computer) connects to the SSD 102via a communication interface 114 conforming to a storage interfacestandard. The SSD 102 functions as an external mass storage device ofthe host device 112. Standards such as SATA (Serial Advanced TechnologyAttachment), SAS (Serial Attached SCSI), PCIe (Peripheral ComponentsInterconnect Express) are examples of the communication interfacestandard between the SSD 102 and the host device 112.

The SSD 102 includes a flash memory controller 116, a random accessmemory (RAM) 118, and NAND memory devices 120. The flash memorycontroller 116 includes a communication interface 114, a flashtranslation layer (FTL) 122, a processor 124, a static random accessmemory (SRAM) 126, a read-only memory (ROM) 128, and a NAND controller130. The NAND controller 130 includes a first bank scheduler 104 havingfirst decision logic 106 and second bank scheduler 108 having seconddecision logic 110. The NAND devices 120 includes one or more devices,each device composed of multiple banks of die coupled to the NANDcontroller 130 by a channel. The NAND devices 120 of FIG. 1 include afirst NAND device group 132 having a first bank (Bank1 0) 134 and asecond bank (Bank1 1) 136 coupled to the first bank scheduler 104 by afirst channel 138, and a second NAND device group 140 having a firstbank (Bank2 0) 142 and second bank (Bank2 1) 144 of devices or dies,which are coupled the second bank scheduler 108 by second channel 146.

The NAND devices 120 are nonvolatile (non-transitory) NAND memorydevices (e.g., first NAND device group 132 and second NAND device group140) configured to store data read and written from and into the hostdevice 112. The flash memory controller 116 performs (executes) datatransfer control on the SSD 102. The RAM 118 temporarily stores datatransferring between the host 112 and the NAND devices 120 by the flashmemory controller 116. The RAM 118 functions as a data cache memory ofthe NAND devices 120, and may employ dynamic random access memory(DRAM), ferroelectric random access memory (FeRAM), magnetoresistiverandom access memory (MRAM), and the like.

The processor 124 in the flash memory controller 116 executes boot codein the ROM 128, transferring control to FTL firmware running in the SRAM126 to manage the interchange of data between the host interface 114 andthe NAND controller 130 via the RAM 118.

The flash memory controller 116 interfaces to the NAND devices 120 byone or more flash memory buses, called channels. For simplicity, FIG. 1shows only two channels, the first channel (CH0) 138 and the secondchannel (CH1) 146, although there may be 8, 16, or more channels. Oneach channel, there may be multiple NAND memory devices (for example,first NAND device group 132 and second NAND device group 140) withmultiple memory dies within each device. Dies or devices may be groupedtogether into units (i.e., banks) which are independently selectableusing chip enable signals. For simplicity, FIG. 1 shows only two banksper channel, Bank 0 and Bank 1, although there may be 8, 16, or morebanks per channel.

The NAND controller 130 includes a bank scheduler (104, 108)corresponding to each of the channels, first channel (CH0) 138 andsecond channel (CH1) 146. First bank scheduler 104 controls thescheduling of memory commands issued to first bank (Bank1 0) 134 andsecond bank (Bank1 1) 136 on first channel (CH0) 138. Second bankscheduler 108 controls the scheduling of memory commands issued to firstbank (Bank2 0) 142 and second bank (Bank2 1) 144 on second channel (CH1)146.

First logic block 106 and second logic block 110 provide bit operationand support logic for bank selection within the first bank scheduler 104and the second bank scheduler 108, respectively, in the NAND controller130. The first logic block 106 and the second logic block 110 arehardware implemented logic blocks including logic gates and directhardware inputs indicating statuses of the devices in the first NANDdevice group 132 and second NAND device group 140 and the first channel(CH0) 138 and the second channel (CH1) 146. The logic blocks 106 and 110enable the bank schedulers 104 and 108 to schedule commands beingtransmitted to the banks in the first NAND device group 132 and secondNAND device group 140 in order to make efficient use of the firstchannel (CH0) 138 and the second channel (CH1) 146.

The first bank scheduler 104 and the second bank scheduler 108 use firstlogic block 106 and the second logic block 110, respectively, todetermine an order in which commands from the host 112 are transmittedto the banks of the NAND devices 120 over the first channel (CH0) 138and the second channel (CH1) 146. Hardware-implemented logic blocks arebetter able to quickly determine an appropriate next bank for commandtransmission over a shared channel, because they are able to efficientlyhandle a multitude of variables and conditions, increasing theefficiency of command processing and scheduling.

A command cycle consists of a sequence of periods or zones. The firstperiod is an address and command setup, followed by a data transfer incases of a write command. This is followed by an inactive period inwhich the command is internally processing in the bank. The final periodis a status or response from the bank. Of these periods, the inactiveperiod is the only period where there is no bus data transfer activity.Bank interleaving allows commands to be sent to other banks over thesame channel during this period of inactivity.

Because some commands, for example a write command, includes ahigh-latency inactive period, a write command can be sent over the firstchannel CH(0) 138 to first bank (Bank1 0) 134, and additional commandscan be sent over the channel to other banks while the first bank (Bank10) executes the command. Interleaving the commands transmitted over thechannel in this way enables the SSD 102 to process commands moreefficiently because multiple banks can be independently executingcommands simultaneously while sharing a single channel. Though only onecommand can be transmitted over the channel at a time, additionalcommands can be sent to other banks in sequence while a first bank isbusy executing a command as long as the bus data transfer zones of thecommands cycles do not overlap.

For a write command transmitted to the first bank (Bank1 0), data to bewritten is presented from the NAND controller 130 to the first bank(Bank1 0) 134 over the first channel (CH0) 138, and the first bank(Bank1 0) 134 begins to write the data. While the data is being written,the first bank (Bank1 0) 134 is busy, but the first channel (CH0) 138may not be busy. More particularly, writing data may take up to 2 ms tocomplete, during which the first channel (CH0) 138 is not busy. Thefirst logic block 106 determines the amount of latency time for thefirst bank (Bank1 0) 134 during the data writing execution anddetermines if an additional command can be sent to another bank over thefirst channel (CH0) 138 during this time, thereby increasing efficiencyby minimizing the time that the first channel (CH0) 138 is idle.

The second bank scheduler 108 functions similarly. The second bankscheduler 108 uses the second logic block 110 to determine an order inwhich commands from the host 112 are transmitted to the banks of theNAND devices 120 over the second channel (CH1) 146. In the same way thatthe first bank scheduler 106 schedules commands over the first channel(CH0) 138 to maximize the efficiency of the command execution, thesecond bank scheduler 108 schedules the commands to the NAND devices 120in the second NAND device group 140 over the second channel (CH1) 146.Multiple bank schedulers operating in tandem in a single SSD 102 allowthe NAND devices 120 in the SSD 102 to efficiently process commands fromthe host 112 in parallel.

For example, while the data is being written by the first bank (Bank1 0)134, the first logic block 106 determines that there are 2 ms in whichthe first bank (Bank1 0) 134 cannot accept additional commands, butother banks in the first NAND device group 132 may accept commands overthe first channel (CH0) 138. The first logic block 106 may set a timerto indicate that there are 2 ms remaining in the latency period for thefirst bank (Bank1 0). The first logic block 106 includes logic thatconsults a queue of commands for additional banks in the first NANDdevice group 132 and selects a bank to which a subsequent command shouldbe transmitted over the first channel (CH0) 138 while the first bank(Bank1 0) 134 is writing data. The subsequent command to be transmittedmay be selected by the bank scheduler 104 based on a plurality ofhardware signals related to the status of the queues and the status ofthe banks. The subsequent command may be a read command to betransmitted to the second bank (Bank1 1) 136. The first bank scheduler104 then transmits the read command to the second bank (Bank1 1) 136over the first channel (CH0) 138. The read command is executed by thesecond bank (Bank1 1) 136 and a completion signal may be transmittedback over the first channel (CH0) 138. A completion signal for the writecommand being executed by the first bank (Bank1 0) 134 may then bereceived over the first channel (CH0) 138, or additional commands may betransmitted to the second bank (Bank1 1) 136 based on the determinationsof the first logic block 106.

FIG. 2 shows a block diagram 200 of a single channel's bank scheduler204 (e.g., corresponding to bank scheduler 104 or 108 described above)with decision logic, according to an embodiment. The block diagram 200includes the bank scheduler 204 having a logic block 206. The bankscheduler 204 is coupled to the NAND devices 232 by a shared NAND buschannel interface 238. The block diagram 200 also includes a bank queue250 divided into normal priority queues (e.g., normal queue 254 denotedby “N”) and priority queues (e.g., priority queue 256 denoted by “P”)for each of the banks of NAND devices 232. A head of each queue 252including a head of the normal queue 258 and a head of the priorityqueue 260 are presented to the bank scheduler 204 into a logic block 206as a status of the queue 262. Also presented to the logic block 206 is aready/busy signal 264 for each bank in the NAND devices 232 and a timer266 which indicates a time remaining in execution of a commandtransmitted to a bank in the NAND devices 232.

The bank scheduler 204 including the logic block 206 accepts variousinputs into the support logic instantiated in hardware, including theready/busy signal 264, timer 266, and status of the queue 262. The bankqueue 250 includes a plurality of command queues for the banks of theNAND devices 232 serviced by the bank scheduler 204. Each bank has anassociated normal queue 254 and priority queue 256 which function as alist of commands from the host sorted into normal and priority commands.For example, in FIG. 2, eight banks of devices or flash memory dies areshown, including a first bank 234 and a second bank 236, andaccordingly, 16 bank queues 250 are shown including eight normalpriority queues (e.g., normal priority queue 254) and eight priorityqueues (e.g., priority queue 256). Whether a command is a normal commandthat should be entered in the normal priority queue 254 or a prioritycommand to be entered into the priority queue 256 may be instructed bythe host or may be based on the type of command. Each queue contains alist of commands to be transmitted to the associated bank of dies in theNAND devices 232 over the common NAND bus channel interface 238. Thefirst command in each queue is in the head of the queue 252 where it isshown as a request packet (RP). The head of the normal priority queue258 contains normal priority command 259. The head of priority queue 260contains priority command 261. The request packet may be any appropriateNAND operation command, including read, write, erase, or other commandsto be transmitted to one of the banks. In some implementations, thelogic block 206 accepts additional inputs such as a current power usageof the NAND devices.

The first command in each queue in the head of the queue 252 isavailable to the logic block 206 in the bank scheduler 204 as the statusof the queue 262. The input of the commands in the head of each queue252 is received by the logic block 206 as the status of the queue 262and enables the logic block 206 to determine to which bank a nextcommand should be scheduled over the NAND bus channel interface 238 inwhich order. The logic block 206 includes logic which gives preferenceto commands which are queued in the priority queues (e.g., head ofpriority queue 260). In some implementations, the logic block 206includes logic that preferences all commands queued in the priorityqueues (e.g., priority command 261) before transmitting any normalpriority commands (e.g., normal priority command 259). In such asituation, normal priority commands (e.g., normal priority command 259)are sent only when there are no commands queued in the priority queues.In some implementations, normal priority commands (e.g., normal prioritycommand 259) and priority commands (e.g., priority command 261) aretransmitted over the NAND bus channel interface 238 in a mixed orderdependent on other inputs to the logic block 206.

In some implementations, the logic block 206 includes logic thatsuspends the processing of normal priority command (e.g., normalpriority command 259) on a bank in order to transmit and execute apriority command (e.g., priority command 261) that has been queued in apriority queue (e.g., priority queue 256 and head of priority queue 260)for the same bank after the normal priority command has beentransmitted. For example, if the normal priority command 259 is an“Erase” command, and there is no priority command 261 in the head of thepriority queue 260, the logic block 206 will transmit the “Erase”command to the first bank 234 over the common NAND bus channel interface238. The logic block 206 sets the timer 266 for a predetermined timeperiod, which may correspond to a period of time typically needed forexecution of the “Erase” command. While the logic block 206 continues toschedule commands to other banks, a priority command 261 such as a readrequest may be placed in the head of the priority queue 260 for thefirst bank 234. If the timer 266 associated with the “Erase” commandexecuting on the first bank 234 has not yet expired, the logic block206, can issue a “Suspend Erase” command to the first bank 234 forpausing execution of the “Erase” command. After the “Suspend Erase”command has been received and/or confirmed by the first bank 234, thelogic block 206 then issues the priority “Read” command to the firstbank 234. The “Read” command is then executed by the first bank 234.After the “Read” command has been executed, if there are no additionalpriority commands 261 in the priority queue 256, the logic block 206 orbank scheduler 204 may instruct the first bank 234 to “Resume” thesuspended erase operation.

A command that has been determined by the bank scheduler 204 to betransmitted next is transmitted over the NAND bus channel interface 238to the appropriate bank of devices or flash memory dies 232 asdetermined by the logic block 206. The command is transmitted with aflash bank command sequence 270. The flash bank command sequence 272includes a first phase “CE on” 272 in which the chip enable for theselected bank of devices or flash memory dies 232 is transmitted to theselected device. The flash bank command sequence 272 also includes asecond phase “Cmd” 274 in which the command code and memory address forthe particular command is given. For example, the command may be a readrequest, a write request, an erase request, or any other suitablerequest. After sending the command to the selected device, the logicblock 206 sets the timer 266 to a predetermined time period based on thecommand type (e.g., a read, write, erase, etc.), the predetermined timeperiod corresponding with a period of time associated with the typicalexecution of the transmitted command type. Depending on the commandtransmitted, the flash bank command sequence 270 may include a thirdphase “Data” 276 in which data is presented over the NAND bus channelinterface 238 to the selected device (for a write command) or by theselected device (for a read command). For certain command types, aninactive phase (not shown) in which no information is transmitted by theselected device over the channel occurs before or after the “Data” 276phase. For example, for a write command an inactive phase occurs afterthe “Data” 276 phase, and for a read command an inactive phase occursbefore the “Data” 276 phase. Erase operations might not have the thirdphase “Data” 276. The selected device within the bank of devices thenexecutes the command. When the timer 266 has expired for the executionof the transmitted command, the logic device 206 requests a status fromthe selected device on the first bank 234, and the selected devicetransmits a signal or suitable response to the logic block 206 for theparticular command transmitted. Alternatively, the logic block 206 maydetermine the status of the operation from the ready/busy signal 264 ofthe selected device. By requesting the status from the selected device,the logic block 206 transmits a request for the status only when thereis an appropriate amount of time in the schedule of commands to bothrequest and receive the status. In some embodiments, the method by whichthe logic block 206 determines a status of the operation may beselectively configured in the supporting hardware. After the command hasbeen completed and the status of the operation has been transmitted tothe logic block 206, the flash bank command sequence 270 then includes afourth phase “CE off” 278 in which the chip enable for the particulardevice is turned off.

When a command is transmitted to one of the bank of devices 232 (e.g.,to first bank 234) via the NAND bus channel interface 238, the logicblock 206 determines whether a subsequent command should be transmittedand also determines to which of the banks the subsequent command shouldbe transmitted to. The logic block 206 continuously has access to theupdated inputs from the ready/busy signal 264, timer 266, and status ofthe queue 262 for each bank. A series of logic gates allows the logicblock 206 to determine the schedule of the commands in order toefficiently distribute and execute the commands to each of the banks.The logic gates also allow the logic block 206 to maintain a consistencyof execution of the scheduled commands.

The logic block 206 determines the order in which banks will receive acommand via the NAND bus channel interface 238. The bank scheduler 204determines the order in which the commands in a queue associated with aparticular bank will be transmitted to the associated bank. For example,the logic block 206 may determine that the first bank 234 is notcurrently busy and is the next bank to which a command should betransmitted. The bank scheduler 204 then determines which command willbe transmitted to the first bank 234, based on the commands at the headof the priority queue 260 and the head of the normal priority queue 258.Each bank has its own pair of priority queue 256 and normal priorityqueue 254 from which the logic block 206 determines an order ofcommands. As an example, if the priority command 261 at the head of thepriority queue 260 for the first bank 234 is a write command, and anormal priority command 259 at the head of the normal priority queue 254for the first bank 234 is a read command, the logic block 206 of thebank scheduler 204 gives preference to the priority command 261 and thebank scheduler 204 transmits the priority write command 261 to the firstbank 234 over the NAND bus channel interface 238 by transmitting thefirst command “CE on” 272 and the second command “Cmd” 274 to the firstbank 234. In this case, the second command “Cmd” 274 is a command toexecute a write program. The bank scheduler 204 then transmits the thirdcommand “(Data)” 276 (which includes the actual data to be written) overthe NAND bus channel interface 238 to the first bank 234. While thefirst bank 234 is executing the write command, the ready/busy signal 264of the first bank 234 will signal that the first bank 234 is busy. Insome embodiments, the ready/busy signal 264 is high when the first bank234 is ready to receive a command and low when the first bank 234 isbusy executing a command. The timer 266 corresponding to the first bank234 will also be set to show that the first bank 234 is busy executing acommand and the expected duration before the command is completed. Insome embodiments, the time on the timer may represent an estimate on thetime needed to complete a command, and may be based on the type ofcommand being executed. Alternatively, the timer may represent anaverage time that it takes for the type of command to be executed,and/or may be updated based on the particular command and amount and/ortype of data to be written or read.

While the first bank 234 writes the data, the logic block 206 uses theinputs including the ready/busy signal 264, timer 266, and status of thequeue 262 for the first bank 234 and all other banks to determine ifadditional commands can be transmitted over the channel before the firstbank 234 completes the data writing, effectively interleaving commandsto make efficient use of the channel. The logic block 206 selects thenext bank for which a command should be transmitted and schedules thenext appropriate command to the selected bank in the device based onwhich banks are busy and the status of each of the queues.

The inputs to the logic block 206 may indicate that there is a normalpriority read command 283 at the head of the normal priority queue 282for the second bank 236. The logic block 206 also has the informationfrom the various ready/busy signals 264 and timers 266, and maydetermine that the first bank 234 is busy and will be busy executing theread request for the next 2 ms, but the second bank 236 is not currentlybusy. If there are no priority commands in the priority queue for thesecond bank 236, the logic block 206 determines that the normal priorityread command 283 should be transmitted to the second bank 236 over theNAND bus channel interface 238 while the first bank 234 continues towrite the data. If there is a priority command 281 in the head of thepriority queue 280, the logic block 206 will instead give preference tothe priority command 281 and transmit the priority command 281 to thesecond bank 236.

The scheduling of priority commands scheduled for different banks ishandled by the logic block 206 based on the input supporting logic thatmay indicate an order that banks should be scheduled in. In someimplementations, the logic block 206 may include logic that determinesan estimated time that the priority commands at the head of priorityqueues associated with each bank will take to transmit and/or executeand determine if the command can be transmitted during the waitingperiod during which the first bank 234 is writing the data. The logicblock 206 is continually requesting and receiving updated inputs fromthe NAND device banks 232 and from the heads of the queues 252, andmaking determinations based on these inputs. Additional inputs to thelogic block 206 may also influence how the logic block 206 makesdeterminations in certain situations, for example whether all prioritycommands are dealt with first or if normal priority commands aretransmitted if they are the only command that fits in a given schedule.

The first bank 234 therefore has a status which changes as it receives acommand, is busy executing a command, and has completed a command. Forexample, once the command has been executed, the status indicates thatthe first bank 234 is ready for a next command. The status of the firstbank 234 is apparent to the logic block 206 via the ready/busy signal264 and the timer 266. The logic block 206 transmits a command to thefirst bank 234, sets the timer 266 for the command and waits and/orchecks the timer 266 until the timer 266 is expired, checks whether thefirst bank 234 has a ready/busy signal 234 that indicates that it isready to receive a command, and if not, the logic block 206 repeats thecheck. Meanwhile, the logic block 206 also checks the ready/busy signal264 and timer 266 of the other banks, determines the order in whichbanks should be scheduled, and transmits commands to these banks ifready and if the command can be scheduled with the currently executingcommands.

FIG. 2 shows 16 bank queues 250 corresponding to 8 banks of devices 232and bank having its own chip enable (e.g., CEO 268). Each of the 8 banksis composed of an array of dies (shown as four dies) which have a singlechip enable. This is an example of a four-way interleaving configurationin which four logical unit number (LUN) NAND dies are grouped into abank and have only one chip enable. Such a configuration allows asmaller number of chip enables to be used on a single channel to accessa large number of individual dies. In some implementations, the bankqueues 250, chip enables, and banks of devices 232 may be interleaved inother ways.

FIG. 3 shows a block diagram 300 that illustrates the direct inputs tothe bank scheduler 304, according to an embodiment. The bank scheduler304 includes the logic block 306, which comprises hardware-based supportlogic. The support logic uses various direct inputs to determine anoptimal schedule of commands to a plurality of devices over a singlechannel. The inputs to the logic block 306 are illustrated in blockdiagram 300 and include a priority command queue (PCMDQ) 356, a normalcommand queue (NCMDQ) 354, a timer status 366, a NAND status 364 (whichmay be a ready/busy signal for a bank), an operation request (ORR) 386(which includes the currently executing command on a particular bank),and an operation status 388 which specifies the inputs to the logicblock 306 and the operation logic to be used in scheduling the commands.

The logic block 306 also includes an external input which is an inputrequest (IR)(n) 384 that can be used to externally control how the logicblock 306 will determine a schedule of commands in particularsituations. For example, the input request (IR)(n) 384 can be changedexternally to enable the logic block 306 to suspend normal prioritycommands in order to transmit priority commands. If needed, additionalinput request (IR)(n) 384 can be input into the logic device 306. Theinput request (IR)(n) may include multiple rules (n rules) in an inputregister (IR) which are applied by the logic block 306 according to aparticular order to quickly select a command to transmit. The operationstatus (OSR) 388 sets the level of operation specifying the inputs fromthe priority command queue 356, a normal command queue 354, a timerstatus 366, a NAND status 364, and the input request (IR)(n) 384 to beused. The bank scheduler 304 has an n-level of decision priority basedon the number (n) of input request (IR)(n) 384 rules, which are meant toallow the logic block 306 to quickly determine a selected command toschedule from many combinations of commands and banks. Once theoperation status (OSR) 388 has set the inputs, the logic block 306attempts to determine a bank for which a command operation should bescheduled according to a first input request (IR)(n) 384 rule. If a bankcannot be determined based on the first input request (IR)(n) 384 rule,a second input request (IR)(n) 384 rule can be applied and the logicblock 306 can again attempt to determine a bank based on the secondinput request (IR)(n) 384 rule. In this way, the command schedulingprocess can be continued throughout all possible cases. The logic block306 may accept additional inputs such as a current power usage of theNAND devices or SSD. When a set of inputs and input request (IR)(n) tothe logic block 306 result in a “hit” (i.e., successfully identify abank for which a command should be scheduled next), the logic block 306stops and the operation status (OSR) 388 reports the result. If there isno “hit” after cycling through the inputs and input request (IR)(n), theoperation status (OSR) 388 reports no hit.

After a bank for which a command operation should be determined isselected by this process, the logic block 306 reports the bank for whicha next command should be scheduled. The bank scheduler 304 then usessupporting logic and rules to determine the command that should beexecuted on the selected bank and transmit the command to theappropriate bank.

FIG. 4 shows a flow chart 400 that illustrates a process for schedulingqueue items using a bank scheduler with hardware support logic,according to an embodiment. At step 402, the input registers are set forprocessing. The input registers include a list of input request ruleswhich the logic block uses to determine a next bank to which a commandfrom the queues should be scheduled. The input register may include aprioritized order of input request rules, which determines the order forapplying the rules. Setting the input registers for processing includesselecting the first priority rule which the logic block should firstattempt to apply to determine the command from the queues that should bescheduled to be transmitted next over the channel. At this step, thelogic block also receives other internal direct inputs that allow thelogic block to determine the selected bank. For example, internal directinputs may include the queue status for priority and normal queues foreach bank, a status of each bank in a ready/busy signal and a timer forany commands currently being executed on a bank. Based on these inputsand the first priority rule of the input register, the logic blockattempts to determine to which bank the next command should bescheduled.

At step 404, the operation request (OPR) is set to process a selectedoperation. The operation request is an external input to the logicblock. The operations request allows the logic block to attempt todetermine a next appropriate bank according to the current rule of theinput register. Also at step 404, the following are set: (1) the inputsfrom the queue status for priority and normal queues for each bank, (2)a status of each bank in a ready/busy signal, (3) a timer for anycommands currently being executed on a bank, and (4) the rule to beapplied by the supporting logic.

At step 406, the process of step 404 is repeated for n levels ofpriority selection. At this step, if the attempt by the logic block atstep 404 to determine a next appropriate bank was unsuccessful, the ruleindicated by the input register is changed and the logic block makesanother attempt to determine a next appropriate bank. This is repeatedfor n rules of the input register until the logic block is successful inidentifying a bank to which a command should next be scheduled.

At step 408, the logic block outputs a decision to the operation status(OSR) based on the received direct inputs and the determination fromsteps 404 and 406 and proceeds to read the operation status to determinewhich bank should have a next command scheduled. The operation status(OSR) may report a “hit” if applying the combination of inputs and ruleresults in a determined bank to be scheduled next. If the combinationyields no result, the operation status (OSR) reports “no hit”. After theoperation status (OSR) reports a bank to be scheduled next, the bankscheduler determines based on a supporting logic and rules which commandfrom the command queues for the chosen bank should be transmitted to thebank next, and transmits the chosen command to the chosen bank.

FIG. 5 shows a timing chart 500 illustrating an operation time of eachbank according to an embodiment. The timing chart 500 includes a list ofthree banks, each with its own timing track, a first bank (Bank 1) 590with first timing track 591, a second bank (Bank 2) 592 with secondtiming track 593, and a third bank (Bank 3) 594 with third timing track595. The first bank 590, second bank 592, and third bank 594 share acommon channel over which data and commands may be transmitted. However,each bank has its own associated chip enable. Each of the three bankscan operate independently in parallel, but data can only be transmittedto one bank at a time over the common channel. The shaded periods ineach of the timing tracks represents the transmission of data over thechannel to the respective bank. As shown, shaded periods do not overlap,as only one command or response signal can be transmitted over thechannel at a time.

The timing chart 500 shows an example outcome in which a logic blockoperating on a bank scheduler is determining an optimal interleaving ofcommands over the channel. In the example, a first write command 596 istransmitted over the channel to the first bank 590. The first writecommand 596 includes a chip enable signal, a write command and the datato be written. The first write command 596 is transmitted from thecommand scheduler to the first bank 590 over the channel. After thefirst write command 596 is transmitted and the data is presented, thereis a period 597 in which the first bank 590 is busy executing the writecommand. No further commands can be sent to the first bank 590 at thistime. However, the channel can still be used by the command scheduler totransmit additional commands to other banks.

In the example illustrated in FIG. 5, the hardware logic blockdetermines, based on inputs from the banks and from the command queues,that there is sufficient time left in the period 597 that additionalcommands can be sent over the same channel without overlappingtransmissions. The logic block determines based on hardware input andinternal logic that the second bank 592 should be scheduled next. Thecommand scheduler determines that the next command to be sent to thesecond bank 592 is a first read command 598. The first read command 598is transmitted over the same channel to the second bank 592, and thesecond bank 592 executes the first read command 598.

The hardware logic block then determines, based on inputs from the banksand from the command queues, that there is still sufficient time left inthe period 597 during which the first bank 590 is still executing thefirst write command 596, that a third command can be sent over thechannel without overlapping with other commands. The logic blockdetermines that the third bank 594 should be scheduled next. The commandscheduler determines that the next command to be transmitted to thethird bank 594 is a second write command 599. The second write command599 is then transmitted over the channel to the third bank 594 includinga chip enable signal, a write data command and the data to be written.After the data has been presented to the third bank 594, the third bankproceeds to execute the second write command 599 by writing the data.During this period 585, the third bank 594 is busy writing the data andno further commands can be transmitted to the third bank 594. However,additional commands can be sent over the channel to other banks.

During the period 585 in which the third bank 594 is busy writing thedata, a first completion signal 587 is received over the channel at thehardware logic block indicating that the first bank 590 has completedthe execution of the first write command 596. In some implementations,the completion signal 587 is received in response to a request for astatus response by the hardware logic block. In some implementations,the request for a status response by the hardware logic block is a checkof a timer set for a particular command execution time and a check of anassociated ready/busy signal for the device. The first bank 590 is nolonger busy and new commands can be transmitted to the first bank 590.

The hardware logic block next determines based on the status of thebanks, the status of the queues, and current rules regarding thepriority ordering of commands, that the next bank to be scheduled to isthe second bank 592. The bank scheduler determines that the next commandto be transmitted to the second bank 592 is a second read command 589.The second read command 589 is transmitted over the channel to thesecond bank 592. As shown, the second read command 589 does not overlapwith any other transmitted commands or completion signals. Finally,after the second read command 589 has been transmitted, a secondcompletion signal 579 is received over the channel at the commandscheduler indicating that the third bank 594 has completed the executionof the second write command 599.

Though FIG. 5 shows an example in which read and write commands aretransmitted to banks over the channel, any NAND command may betransmitted over the channel. Additionally, the hardware logic block maydetermine a scheduling order for more than three banks of NAND die, forexample, a hardware logic block on a command scheduler may schedule thecommand order over a channel for eight or sixteen banks of NAND devices.

Employing a hardware-based logic block to determine the optimal orderfor commands to be distributed enables the command scheduler to schedulethe commands for transmission to a larger number of banks. Thehardware-based logic uses logic gates and direct hardware inputs andsignals to make a determination of a next bank for which to schedule acommand to be transmitted over a channel more quickly than asoftware-based system. A software-based system for selecting a bankscheduling order becomes unwieldy and performance suffers when it isexpanded to include a large number of banks, such as eight or 16 banks,each having multiple hardware signals which influence the determinationof whether and when a command may be transmitted to the bank.

Other objects, advantages and embodiments of the various aspects of thepresent invention will be apparent to those who are skilled in the fieldof the invention and are within the scope of the description and theaccompanying Figures. For example, but without limitation, structural orfunctional elements might be rearranged consistent with the presentinvention. Similarly, principles according to the present inventioncould be applied to other examples, which, even if not specificallydescribed here in detail, would nevertheless be within the scope of thepresent invention.

What is claimed is:
 1. A memory system comprising: a memory controllerhaving a bank command scheduler implemented in a hardware logic block,the hardware logic block including a plurality of direct inputs; a firstmemory bank having a first I/O bus; a second memory bank having a secondI/O bus; and a channel connecting the first I/O bus and the second I/Obus to the memory controller, the channel configured to: transmit databetween the first memory bank and the memory controller and between thesecond memory bank and memory controller, and transmit a plurality ofcommands from the memory controller to the first memory bank or thesecond memory bank, wherein the hardware logic block is configured to:make a first selection, based on the plurality of direct inputs, of anorder in which to output commands to the first memory bank and thesecond memory bank over the channel; output a first command to the firstmemory bank based on the first selection; for the first command,estimate a time remaining in execution of the first command, wherein theestimated time remaining is one input of the plurality of direct inputs;make a second selection of a second command based on the estimated timeremaining of the first command; and output the second command to thesecond memory bank over the channel based on the second selection. 2.The memory system of claim 1, wherein the bank command schedulerincludes a first queue of commands to be distributed to the first memorybank and a second queue of commands to be distributed to the firstmemory bank.
 3. The memory system of claim 2, wherein the first queue ofcommands comprises high priority commands and the second queue ofcommands comprises normal priority commands.
 4. The memory system ofclaim 1, wherein the hardware logic block includes an external input ofa first rule for determining an order in which to output commands to thefirst memory bank and the second memory bank over the channel.
 5. Thememory system of claim 4, wherein when the hardware logic block isunable to determine an order in which to output commands to the firstmemory bank and the second memory bank over the channel based on theplurality of direct inputs and the external input of a first rule, a newexternal input of a second rule is assigned.
 6. The memory system ofclaim 1, wherein the plurality of direct inputs includes informationrelated to the first memory bank and the second memory bank, and whereinthe plurality of direct inputs related to the first memory bank and thesecond memory bank includes the estimated time remaining of the firstcommand and an estimated time remaining of the second command as a timerstatus of each of the first memory bank and the second memory bank,respectively.
 7. The memory system of claim 6, wherein the first commandtransmitted by the bank command scheduler to the first memory bank overthe channel is a write command.
 8. The memory system of claim 7, whereinthe hardware logic of the bank command scheduler determines, based onthe timer status of the first memory bank, whether an additional commandcan be sent over the channel to the second memory bank during theestimated time remaining of the first command.
 9. The memory system ofclaim 8, wherein the bank command scheduler transmits a read command tothe second memory bank over the channel during the estimated timeremaining of the first command based on the second selection of thehardware logic.
 10. The memory system of claim 9, wherein the bankcommand scheduler receives a command completion status transmitted bythe first memory bank in response to a request for a command completionstatus.
 11. The memory system of claim 1, wherein the plurality ofdirect inputs related to the first memory bank and the second memorybank includes at least one of commands queued to be executed in each ofthe first memory bank and second memory bank, commands currently beingexecuted in each of the first memory bank and second memory bank, and astatus of the first memory bank and second memory bank.
 12. The memorysystem of claim 1, wherein the first memory bank and second memory bankcomprise a NAND device.
 13. A method of scheduling a distribution ofcommands to a first and a second memory bank, the method comprising:making a first selection, by a hardware logic block, of a first commandto send to a first NAND bank over a channel, transmitting the firstcommand to the first NAND bank over the channel; estimating, by thehardware logic block, a time remaining in execution of the first commandfrom a plurality of direct hardware inputs; making a second selection,by the hardware logic block, of a second command to send to a secondNAND bank over the channel, based on the estimated time remaining of thefirst command; and transmitting the second command to the second NANDbank over the channel.
 14. The method of claim 13, wherein the selectingthe first command and the selecting the second command is based on afirst rule from a plurality of rules for determining an order ofcommands.
 15. The method of claim 14, further comprising selecting asecond rule from the plurality of rules, and attempting aredetermination of a first command when the selecting the first commandis unsuccessful.
 16. The method of claim 15, wherein the plurality ofdirect hardware inputs includes information related to the first NANDbank and the second NAND bank, and wherein the plurality of directhardware inputs related to the first NAND bank and the second NAND bankincludes the estimated time remaining of the first command and theestimated time remaining of a second command as a timer status of eachof the first NAND bank and the second NAND bank, respectively.
 17. Themethod of claim 16, wherein the first command is selected from a firstqueue or a second queue associated with the first NAND bank.
 18. Themethod of claim 17, wherein the first queue is a priority queue and thesecond queue is a normal priority queue.
 19. The method of claim 18,further comprising selecting, based on the hardware logic block, a thirdcommand to be transmitted to a third NAND bank over the channel duringan overlap of the estimated time remaining of the first command and theestimated time remaining of the second command.
 20. A method ofdetermining a bank scheduling order for a NAND device having a pluralityof banks with a shared channel, the method comprising: setting an inputregister to a first priority rule; setting an operation request todetermine a first status of (1) the plurality of banks and (2) aplurality of command queues; determining whether the first priority ruleand the operation request is sufficient to select a bank to receive acommand; in response to determining that a bank can be selected toreceive the command according to the first priority rule and theoperation request, checking an operation status for the bank scheduledto receive the command, and transmitting the command to the bank whenthe operation status is not busy; in response to determining that a bankcannot be selected to receive the command according to the firstpriority rule and operation request, setting the input register to asecond priority rule, and setting an operation request to determine asecond status of (1) the plurality of banks and (2) the plurality ofcommand queues, determining that the second priority rule and operationrequest is sufficient to select a bank to receive a second command, andtransmitting the second command to the selected bank.